Morphology of uranium precipitates in silicon

Abstract

Soft errors induced by incident alpha particles have been observed in dynamic Metal-Oxide- Semiconductor Random Access Memory (MOS RAMs), charge coupled devices (CCDs), static MOS RAMs and in bipolar RAMS. The focus of attention has been the occurrence of soft errors as a result of alpha particle decay of uranium present even in trace amounts, typically a few ppm (parts per million), in the silicon substrate along with the devices as well as the packaging material. A study of segregation of uranium in silicon is of special interest since a single alpha particle of approximately 40 MeV that has a track length of about 20 μ in silicon can give rise to nearly 106 electron-hole pairs if all the deposited energy is converted to the generation of excess carriers. If these excess carriers are developed in a critical volume depending upon the density of devices in a product, the effect can be large enough to limit the mean time of failure to a few days or weeks for a 64k memory cell. In order to study the diffusion and segregation characteristics of uranium in silicon, uranium was deliberately ion implemented into bare silicon to a dose of 1014/cm2 and subsequently annealed at several temperatures often encountered in device processing. In this paper, we present TEM results especially pertaining to the morphology of uranium precipitates in silicon. Conventional microstructural observations, using bright field (BF) imaging, stereographic projection analysis using electron diffraction patterns, and projected directions of the precipitate with respect to matrix reflection from silicon onto a stereogram, and moire patterns using phase contrast imaging, were used to determine the morphology of fine uranium precipitates in ion implanted and annealed samples.